diff --git a/cpu/stm32l1/periph/timer.c b/cpu/stm32l1/periph/timer.c
index ac6f190481..03c074b9b8 100644
--- a/cpu/stm32l1/periph/timer.c
+++ b/cpu/stm32l1/periph/timer.c
@@ -56,7 +56,7 @@ int timer_init(tim_t dev, unsigned long freq, timer_cb_t cb, void *arg)
     tim = _tim(dev);
     /* save callback */
     isr_ctx[dev].cb = cb;
-    isr_ctx[dev].cb = arg;
+    isr_ctx[dev].arg = arg;
     /* enable peripheral clock */
     RCC->APB1ENR |= (1 << timer_config[dev].rcc);
     /* reset timer and configure to up-counting mode */